When you add a location to a tweet, you have to option of picking the location from a searchable list of various nearby locations. The search here isn't limited in any way, so you can practically choose any location you wish.
I also have English OS, Firefox using English locale, and I am in the US.
So to force me to use Japanese map service, I went to www.bing.co.jp (same as I do for Google when I explicitly want Japanese results), the select Maps from the top menu. Since you are in Japan, Bing might be sending you there by default.
Then I entered "10 Downing Street, London" as ...